The add-on boot serves for increasing the size of the boot in cars which have a notchback. It comprises two side parts, a rear door and three adapting elements which are put together with the boot lid and the boot edge in such a manner that the car may, if required, be converted into an estate shape, it being possible rapidly to regain the original shape by dismantling. To this end, only a few, small changes are necessary to the previous car body and, as a result, the add-on boot can be used both for new cars and also for many used cars.